We propose to study the role of the cell surface in lymphoid cell differentiation by examining cell surface antigens determined by the Tla region of the mouse. This region controls at least four serologically defined differentiation antigens, Qa.1, Qa.2, Qa.3, and TL. Our approach will be to determine (1) how these cell surface antigens are regulated during lymphoid cell development; (2) how the Tla region genes control these cell surface components; (3) what other cell surface antigens are controlled by the Tla region; and (4) what is the possible significance of the Tla region and the cell surface antigens which it determines on the functioning of the immune response. Our techniques will include transplantation and immunization procedures, serological definition of cell surface antigens by cytotoxicity testing, mixed lymphocyte culture, genetic linkage and allelism studies, mitogen studies, functional assays for killer and helper cell activity.